With declination of fossil fuel reserves and sudden rise and change of oil prices, intensive worldwide attention has been paid to improvement in fuel efficiency of vehicles.
For fuel efficiency improvement, weight reduction design of vehicle bodies and minimization of power loss by reducing frictions at system links have been developed. Further, maximization of output efficiency increases fuel efficiency by improving dynamic characteristics upon exhaustion control of an engine itself. In regard to the improvement of fuel efficiency, efforts have been made to reduce a dynamic load through weight reduction of dynamic components of the engine head.
Among the dynamic components, an engine valve spring is a component that may make a great contribution to fuel efficiency if its weight is reduced because the engine valve directly controls a dynamic load. Conventionally, valve springs are made mainly of CrSi steel with a tensile strength of 1900 MPa or CrSiV steel with a tensile strength of 2100 MPa. Further, attempts have been made to increase the tensile strength to 2550 MPa by adding alloy elements to the CrSiV steel.